1. Field of the Invention
The present invention relates to powder paint composition and a process for forming multi-layer coats containing the coat formed from the powder paint composition.
2. Description of the Prior Art
The powder paint that does not contain any organic solvent in the composition has been popularly adopted in the field having severe solvent restrictions, because the powder paint does not raise environmental pollution during coating. In addition, since the powder paint is easy to recycle and reuse during coating, and thick films can be formed in a lesser number of processes, the powder paint has been noted as a paint suited for world trends of non-pollution and resource conservation.
The powder paint is generally coated by electrostatic coating using a spray gun on a substrate which is grounded. Therefore, the powder paint can form uniform coated film on a flat portion of the substrate, but does not form uniform coated film on a non-flat surface portion of the substrate (particularly, dented parts of irregular profiles, etc.), because the particles of the powder paint do not satisfactorily adhere to the non-flat surface. Also, since the powder paint has a large paint particle size, it is difficult to reduce the irregularity of the coat surface even by melting and baking after coating.
In order to solve the above problem, a process for forming multi-layer coats has been proposed in Japanese Kokoku Publication Sho 56-10397 wherein powder coating is conducted on a substrate by a conventional method to form uniform coat on flat portions of the substrate and then electrodeposition paint is coated on the substrate to form a coated film an the non-flat portions or inner portions of the substrate in which uniform coated film is not generally formed. This process has been actually employed particularly for coating outer and inner plates of automobile car bodies.
However, the above mentioned process does not form a coated film having sufficient thickness on boundary portions between the powder-coated film and electrocoated film, and corrossion resistance is poor on the boundary portions, because electrocoated film does not sufficiently form on the portion where powder coated film is formed, as the electrodeposition is conducted after powder coating.
A large number of processes for improving the corrosion resistance at the boundary portions between the powder-coated film and the electrocoated film have also been proposed (Japanese Kokoku Publication Sho 60-111, Japanese Kokai Publications Hei 9-20663 and 9-20664).
In Japanese Kokoku Publication Sho 60-111, there is disclosed a process for enabling electrodeposition on powder-coated films by using electro-conductive powder paint In Japanese Kokai Publication Hei 9-20663 and 9-20664, there are disclosed processes for applying electro-conductive primer before or after the powder paint coating to secure the film thickness at the boundary portion between the powder-coated film and the electrocoated film.
However, in the former process, the electrodeposition occurs even on the thick film portion of the powder-coated film and the appearance of the powder-coated film is degraded (degraded smoothness) and economy is also degraded due to the use of unnecessary electrodeposition paint.
In the latter process, the high corrosion resistance required for the multi-layer coats is not obtained, because the coating process becomes complicated as the conductive primer coating process is added, the existence of the conductive primer coat impedes high corrosion resistance of electrocoated film, and the conductive primer does not exhibit satisfactory adhesion with the powder paint or the electrocoated film applied thereon.
It is an object of the present invention to provide a powder paint composition that can be applied to the conventional and known multi-layer coat forming process and that can achieve high corrosion resistance at the boundary between powder-coated film and the electrocoated film as well as excellent adhesion with a film formed thereon.
It is another object of the present invention to provide a process for forming multi-layer coats having high corrosion resistance and excellent appearance using the powder paint composition.
A powder paint composition comprising:
a) 30-5 parts by weight of a polyester resin having a hydroxyl value of 50-150;
b) 1-20 parts by weight of a hydroxyl group-containing acrylic resin having a hydroxyl value of 30-150;
c) 1-15 parts by weight of an epoxy resin; and
d) 10-30 parts by weight of a urethodione ring-containing compound, based on 100 parts by weight of solid contents in the powder paint composition, the powder paint composition having an average particle size of 5-40 xcexcm.
The powder paint composition can exhibit the following effects by formulating plural types of resins having high hydroxyl values and a curing agent having a specific structure.
1. It is possible to provide powder-coated film that exhibits high corrosion resistance by forming a network structure in the coat by cross-linking reactions with a hydroxyl group-containing resin, such as polyester resin (a), etc. by a specific curing agent, while the epoxy resin (c) that can provide satisfactory corrosion resistance is contained in the powder paint composition.
2. It is possible to improve the adhesion with the top coat formed thereabove by the existence of the hydroxyl group-containing acrylic resin. In addition, if necessary, a petroleum resin (e) is formulated into the powder paint composition to improve the adhesion.
3. By reducing an average particle size of the powder paint composition, the smoothness and appearance of the powder-coated film can be improved.
The powder paint composition is characterized by the urethodione ring-containing compound (d) as a curing agent, of which a glass transition temperature is higher than that of the conventional blocked isocyanate curing agent (Tg=40-55xc2x0 C.), that is, not lower than 55xc2x0 C. In this context, the present invention can provide a powder-coated film with excellent corrosion resistance.
The second embodiment of the present invention is a process for forming multi-layer coats 10 using the powder paint composition and electrodeposition paint as shown in FIG. 1, comprising
(i) a process for forming a powder-mated film 1 by electrostatically coating the powder paint composition on a metal substrate 3,
(ii) a process for heating the powder-coated film to the temperature at which the powder-coated film is not completely cured,
(iii) a process for dipping the metal substrate with the powder-coated film in an electrodeposition paint bath to form an electrocoated film 2,
(iv) a process for forming multi-layer coats by heating to cure both the powder-coated film and the electrocoated film, and
(v) a process for forming a top coat 4 on the multi-layer coats obtained in the preceding process.
The present invention further provides the multi-layer coats 10 (see FIG. 1) formed by the above-mentioned process.